Vibratory screening machines are well known in the art and are used in a variety of situations where it is desired to remove suspended solids from a slurry. These machines include an screen assembly that is vibrated. The liquid slurry is discharged onto the screen which is typically inclined. Solids in the slurry, larger than the screen size, collect on top of the screen and migrate toward the discharge end. Solids in the slurry smaller than the screen size pass with the liquid through the screen.
A typical application of vibratory screening machines is in the drilling of oil or gas wells where these devices are known as shale shakers. Early shale shakers incorporated a single inclined vibrating layer of hardware cloth having a mesh opening of 1/4-3/8". Drilling mud coming from the well discharged onto the inclined screen. Large shale particles collected on top of the hardware cloth and travelled down the incline into a shale pit. The liquid drilling mud and the bulk of the entrained solids passed into the mud system.
Substantial improvements have been made in vibratory screening machines so very small solids are now capable of being removed from hot drilling mud streams emitting from wells being drilled at substantial depths in the earth. Larger drilling rigs are equipped with sophisticated mud systems that treat the drilling mud to perform its various tasks. A typical large drilling rig includes a shale shaker mounted on a mud tank so the removed solids are discharged into a shale pit adjacent the mud tank and the liquid mud passing through the shale shaker falls into the mud tank where it is treated by monitoring of various properties, by adding various chemicals and by using other solids removal techniques.
When starting the drilling of a land based well, however deep, the surface hole is drilled with a combination of water and bentonite gel which combines with drilled solids to make a native drilling mud. This type mud is not expensive and is not treated in a costly manner. When drilling the surface hole, the screen assemblies on the shake shaker are selected to have rather large mesh so that only fairly large solids are removed from the mud stream. Because the screen assemblies have large mesh screen, they have screen wire of substantial diameter and are accordingly robust and operate satisfactorily for substantial lengths of time.
As the well is deepened, the drilling mud is treated with more expensive chemicals and more care is taken to control the amount and size of solids in the recirculated mud. In the drilling of a typical deep well, one or more strings of intermediate pipe are cemented in the hole to provide protection against blow outs. Typically, more expensive mud types are used following the setting of intermediate strings. For example, it is common in parts of South Texas to drill a well with a water based gel mud until an intermediate string of pipe is set and then change over to an oil based invert emulsion. These oil based emulsions are considerably more expensive than the water based mud used to drill the shallower part of the hole. Considerably more care is taken to remove solids from more expensive muds, of which oil based invert emulsions are typical.
The screen assemblies in shale shakers are accordingly changed during drilling of wells to provide larger mesh, less expensive, more durable screen assemblies when drilling the shallow part of the hole and smaller mesh, more expensive, less durable screen assemblies when drilling the deeper part of the hole. The trend, over time, has been to use finer and finer mesh screens when using expensive muds. The finest screen mesh presently employed in screen assemblies is on the order of 210-250 mesh, which means there are 210-250 strands of wire per inch. A conventional 210 mesh screen will remove solids larger than 74 microns from drilling mud.
There are presently two types of screen assemblies employed in sophisticated vibratory screening machines used as shale shakers. One type employs a rigid metal plate as a support for the screens. A second type employs a perforate plastic sheet or grid and a single screen as a support.
As shown in FIG. 1, a prior art screen assembly 10 includes a fine mesh screen 12, a blinding screen 14, a plastic grid or mesh 16 and a load bearing screen 18. The elements 12, 14, 16, 18 are put into a heated press where the temperature softens the plastic mesh 16 and an applied pressure squeezes the screens 12, 14, 18 into the plastic mesh 16, or vice versa, thereby bonding the layers together to provide a unitary structure. A conventional edge structure or channel 20 is incorporated with the screens so the assembly can be placed into a conventional vibratory screening machine. Screen assemblies of this construction have proved suitable for use in the shallower part of hole where the upper screen 12 is on the order of 120 mesh or coarser. Screen assemblies made in this manner using finer mesh screens have not proved suitable because they are not sufficiently durable, wear out too quickly and are thus too costly.
Disclosures of interest relative to this invention are found in U.S. Pat. Nos. 4,033,865; 4,575,451; 5,221,008; 5,330,057; 5,417,859 and 5,673,797.